(1) Field of the Invention
The present invention relates to a video signal processing system, and particularly to a device for converting an analog television signal into a signal in compliance with a certain standard for digital encoding.
(2) Description of the Prior Art
In recent years, appliances capable of recording low noise video images by digitally processing video signals, such as DVC (Digital Video Cassette) and the like, have become available in the market. In line with the development of such appliances, a device for converting an analog television signal to a digital signal in compliance with a certain digital encoding standard has been increasingly demanded.
A standard for the analog television signal, for example in NTSC system, is specified in EIA standard RS-170. The signal according to this standard comprises, as shown in FIG. 1, a video signal portion that includes a luminance signal and a carrier chrominance signal, and a synchronizing signal portion that is used for synchronizing operation to reproduce a picture in a television receiver. The signal levels for each of the signals are specified to be that, when a signal level of the synchronizing signal is 40, the video signal has a signal level difference between a black level and a peak white level of 100.
A standard for a digital encoding is specified, for example, in ITU-R Recommendation BT601 as a digital encoding standard for a component signal. This standard specifies a sampling frequency, a signal level for quantization, etc. in converting an analog luminance signal and an analog carrier chrominance signal to digital signals. For NTSC system, for example, it is stated that no synchronizing signal is used, and that in the luminance signal a sampling frequency for sampling analog data is 13.5 MHz, and signal levels for quantization are a black level being 16/255 (=28xe2x88x921), and a peak white level being 235/255 when one sample is expressed (converted) by eight bits.
As described above, between these two standards, there is a difference in a video signal level as shown in FIG. 2. In FIG. 2, on the left is a video signal according to the EIA standard RS-170, and on the right is a video signal according to the ITU-R Recommendation BT601. Thus, a need to adjust the difference arises in converting analog video signals to digital signals.
In conjunction with the above, a prior art device for converting an analog television signal to a digital signal in compliance with a digital encoding standard is now described with reference to the figures.
FIG. 3 shows a construction of a prior art analog-to-digital converter. In the figure, there are shown an upstream A-D converter circuit 111, a downstream A-D converter circuit 112, a Y/C separating circuit 12, an upstream synchronizing signal detecting circuit 131, a downstream synchronizing signal detecting circuit 132, a D-A converter circuit 31, an amplifying circuit 32, and a synchronizing signal converting circuit 14. The operation of this prior art device is as follows.
An analog television signal, such as a signal for NTSC system, is supplied from an input terminal 10 into the upstream A-D converter circuit 111, and converted therein to an 8- digital signal. As for signal levels of the output signal from this A-D converter circuit, the black level is made to be, for example, 64/255 and a peak white level is 210/255, as shown in FIG. 4. The upstream synchronizing signal detecting circuit 131 receives the 8-bit digital signal and detects a synchronizing signal from the 8-bit digital signal. The Y/C separating circuit 12 separates a luminance signal and a carrier chrominance signal from the 8-bit digital signal using the detected synchronizing signal, and thereby a composite signal, which contains a synchronizing signal, a luminance signal, and a carrier chrominance signal, is converted into a component signal in which a signal containing the luminance signal and a synchronizing signal is separated from a carrier chrominance signal. The separated luminance signal is converted back to an analog signal by the D-A converter circuit 31, and amplified by the amplifying circuit 32 so that the signal level between the black level and the peak white level becomes 1.5 times larger than that of the input signal. The amplified luminance signal is then converted back to an 8-bit digital signal by the downstream A-D converter circuit. As for signal levels of the output signal from the A-D converter circuit 112, the black level is made to be 16/255 and the peak white level is 235/255, as in the waveform shown on the right of FIG. 2. The downstream synchronizing signal detecting circuit 132 detects a synchronizing signal from the output signal from the amplifying circuit 32. The synchronizing signal converting circuit 14 uses the synchronizing signal received from the synchronizing signal detecting circuit 32 to fix a synchronizing signal portion of the luminance signal to be, for example, 16/255, and output a video portion of the luminance signal without changing. Finally, the carrier chrominance signal and the luminance signal are separately output to a display (not shown), and thus a better picture is attained than a picture obtained in the case where the output signal to the display is such that the carrier chrominance signal and the luminance signal are combined.
According to such a construction described above, an analog television signal, such as a signal for NTSC system, can be converted to a digital signal in compliance with a digital encoding standard. However, the above-described circuit construction has such a drawback that, after converted to a digital signal by the upstream A-D converter circuit 111, the input signal is required to be converted back to an analog signal by in the D-A converter circuit 31 and converted back again to a digital signal by the downstream A-D converter circuit 112, thereby causing the deterioration of a signal-to-noise ratio. Furthermore, to change the signal level of the luminance signal, a plurality of A-D converter circuits as well as a D-A converter circuit are required, therefore the cost is increased.
In view of the above drawbacks, a video signal processing system having an improved signal-to-noise ratio, having a small circuit size, and being cost-effective, is desired. In addition, although the foregoing discussion pertains primarily to a signal processing in an A-D conversion for television pictures according to NTSC system, such as normally used in U.S.A. and Japan, there remain problems caused by amplifying the signals to adjust a difference between the signals each based on a different standard for digital encoding even under other systems. Therefore, there also exists a need to develop a technique that can be suitably applied to such cases.
In addition, as a matter of course, qualities of resulting pictures must not be deteriorated as a result of signal adjustments by amplification and the like. Accordingly, it is also desired to develop a technique that can prevent the deterioration of picture qualities caused by such adjustments and the like processing.
Therefore, it is an object of the present invention to provide a solution to the foregoing problems and drawbacks. According to the present invention, video signal conversion is performed digitally, unlike prior art circuits in which such conversion is performed with analog signals.
This and other objects are accomplished in accordance with the present invention by providing an analog-to-digital converter for converting a video signal comprising:
an A-D converter circuit for converting an analog television signal to a 2n-bit digital signal,
a synchronizing signal detecting circuit for detecting a synchronizing signal in a digital signal received from the A-D converter circuit,
a Y/C separating circuit for separating a luminance signal and a carrier chrominance signal from the digital signal received from the A-D converter circuit utilizing a synchronizing signal supplied from the synchronizing signal detecting circuit,
a synchronizing signal converter circuit for converting a synchronizing signal portion of the luminance signal received from the Y/C separating circuit to a predetermined value, and
a digital amplifying circuit for amplifying a signal level of a signal received from the synchronizing signal converter circuit by a predetermined multiple value in compliance with a standard for communication or a standard for an apparatus using a video signal.
In the above construction, the 2n-bit digital signal may be, for example, an 8-bit digital signal, and the predetermined multiple value by which the signal from the synchronizing signal converter circuit is amplified may be set at, for example, 1.5 times.
In the above construction, the digital amplifying circuit may be constructed by a digital multiplier, or by at least one shifter for shifting the input digital signal a predetermined bit number to the right or to the left and an adder for adding an output signal from the shifter and an original input signal supplied to the digital amplifying circuit.
In addition, a low pass filter for passing a predetermined low frequency component in a digital signal received from the digital amplifying circuit may be provided in the above construction, to eliminate errors such as high frequency caused in the processing and obviously not included in the supplied input video signals.
Further, in the above construction, there may be provided a control section for detecting a predetermined characteristic of an input signal and outputting a control signal corresponding to a detected value and a selector circuit for selecting one of an output signal from the digital amplifying circuit and an output signal from the low pass filter in response to the control signal from the control section.
According to this construction, in consideration of human visual characteristics, the selector circuit appropriately select s and outputs either a signal being closer to an original input signal but having rounding errors, or a signal having smaller rounding errors but having a lower picture quality.
The a control section and the selector circuit may be provided in combination with the digital multiplier, or the shifter and the adder mentioned above.
In addition, the above control section may be a movement detecting control section for detecting a movement in an output signal from the A-D converter circuit or the Y/C separating circuit, and thereby generating such a control signal t hat the selector circuit selects an output signal from the digital amplifying circuit in the case where a detected value is equal to or higher than a threshold value, and selects an output signal from the low pass filter in the case where the detected value is smaller than a threshold value.
According to this construction of the control section, when a video signal to be output is decided as a signal of a moving picture, a signal being close to the original analog signal but having rounding errors is selected to be output, since in the case of moving pictures the rounding errors are not very much noticeable because of such human visual characteristics. On the other hand, when a signal to be output is decided as a signal of a still picture, a low pass filtered signal with a small rounding error is selected to be output, since the rounding error becomes more apparent to human eyes in the case of still pictures.
In addition, the above control section may be a correlation detecting control section for detecting a correlation in an output signal from the A-D converter circuit or the Y/C separating circuit, and thereby generating such a control signal that the selector circuit selects an output signal from the digital amplifying circuit in the case where a detected value is equal to or higher than a threshold value, and selects an output signal from the low pass filter in the case where the detected value is smaller than a threshold value.
According to this construction of the control section, when a video signal to be output is decided to have a weak correlation, a signal being close to the original analog signal but having rounding errors is selected to be output, since in the case of such a signal the rounding error is not much noticeable because of such human visual characteristics. On the other hand, when a signal to be output is decided to have a strong correlation, a low pass filtered signal with a small rounding error is selected to be output, since the rounding error becomes more apparent to human eyes in the case of a signal having a strong correlation.
Further, the above control section may be a movement and correlation detecting control section for detecting both the movement and the correlation in an output signal from the A-D converter circuit or the Y/C separating circuit.